Oxidation of a crystalline silicon substrate results in the formation of a layer of silicon dioxide on the substrate surface. Photolithography can then be used to selectively pattern and etch the silicon dioxide layer to expose a portion of the underlying substrate. These openings in the silicon dioxide layer allow for the introduction ("doping") of ions ("dopant") into defined areas of the underlying silicon. The silicon dioxide acts as a mask; that is, doping only occurs where there are openings. Careful control of the doping process and of the type of dopant allows for the creation of localized areas of different electrical resistivity in the silicon. The particular placement of acceptor ion-doped (positive free hole, "p") regions and donor ion-doped (negative free electron, "n") regions in large part defines the interrelated design of the transistors, resistors, capacitors and other circuit elements on the silicon wafer. Electrical interconnection and contact to the various p or n regions that make up the integrated circuit is made by a deposition of a thin film of conductive material, usually aluminum or polysilicon, thereby finalizing the design of the integrated circuit.
Where it is desired to have semiconductor devices with uncommitted logic gates such that the final logic configuration of the device is determined by the end user, the fabrication process must allow for programming of the device. Programming normally involves adjusting threshold voltages of particular gate transistors located either in or out of a memory row and column matrix. Threshold reduction is achieved by doping the region of the selected depletion devices utilizing ions of a conductivity type the same as that of the MOS transistor's source and drain. Threshold increases are achieved by introducing into the channel region ions of a conductivity type opposite to that of the MOS transistor's source and drain.
Programming methods typically utilize ion implantation to adjust the channel voltage thresholds. In general, the variations among these programming methods involve the number of layers through which ion implantation is performed. In one case, for example, very high energy ion implantation is performed to penetrate the various layers. In another case, low energy ion implantation is performed after etching a deep hole in the deposited layers.
There are a number of types of ions used in such processes; boron is very commonly used. On the other hand, germanium ions have been used; U.S. Pat. No. 5,347,151 to Shirnizu et al, hereby incorporated by reference, describes the implantation of germanium ions under the conditions of the implantation energy 100 KEV and the dose 1.times.10.sup.16 cm.sup.-2. Germanium has also been used to enhance doping with n-type dopants; the use of low-pressure vapor deposition of a germanium containing gas into the silicon layer is described in U.S. Pat. No. 5,316,958 to Meyerson, which is hereby incorporated by reference.
There are a number of problems with the ion implantation approach to programming. Most importantly, there is the problem of metallization over the region to be doped. Metallization will block and prevent proper ion implantation. Furthermore, even if metallization can be avoided in the physical area of ion implant, achieving doping at the required depth requires an ion implantation instrument with very high ion beam energy and high through put capacity. Ion implant equipment required to meet these requirements is expensive.
Additionally, vapor deposition of germanium has been used for doping materials in the field of fiber optic cable. For example, fused silica or multicomponent glasses are formed into cable by a "vapor phase process." In this process, SiCl.sub.4 is introduced as a vapor and oxidized in a flame to form SiO.sub.2 vapor. This vapor is then deposited upon a glass or graphite "bait rod" to form a fiber optic cable. Similarly, GeCl.sub.4 is oxidized in flame, forming GeO.sub.2 vapor to be deposited in the bait rod to serve as a dopant to change the forming fiber optic cable's index of refraction. [See J. P. Powers, An Introduction to Fiber Optic Systems, R. D. Irwin & Asken Assoc., Inc. Boston, Mass. (1993) pp. 459-468.]
This process for doping has a number of disadvantages. Most importantly, applying GeCl.sub.4 results in the formation of some environmentally hazardous and unsafe byproducts including HCl, and chlorine gas. Given these problems, it is important to prepare new germanium-containing compounds which can function as dopants and where the methods for their use are flexible, reliable and environmentally safe.